It is important to hold the path length of a ring laser gyro at an optimum length wherein the principal mode of oscillation is maximum. The principal mode or wave of the laser will oscillate, and the ring cavity will support such wave over a limited excursion from the optimum path length of the laser.
When the path length of the ring laser is optimum, the rate of change of the amplitude of the laser light intensity with respect to the position of any particular mirror is zero. In the prior art, a controlled mirror is dithered at a low frequency and amplitude. The dithering of the mirror causes the amplitude of the envelope of the output signal of the ring laser to be modulated at the dither frequency. The envelope of the output signal is then demodulated at the dither frequency, and the demodulated signal is integrated with the integrated signal used to move the average position of the dithered mirror to produce a maximum amplitude of the output envelope. Such control is typically accomplished by servoing the mirror to a position to dither about the position where the rate of change of the amplitude with respect to the mirror position is zero.
The above-described servo causes the demodulated amplitude of the ripple on the envelope of the output signal to approach its minimum value, when the average amplitude reaches its maximum value. Consequently, minimizing the amplitude of the ripple moves the cavity mirror to its optimum position.
Causing the demodulated amplitude of the ripple on the envelope of the output signal to approach its minimum value causes the cavity to be properly tuned to its principal mode of oscillation only if the starting position of the mirrors is such that the laser oscillation principal mode can be supported. Should the initial position of the mirror be such that it excites a secondary or undesired mode, the mirror must be moved controllably into a different position where it will excite the principal laser mode.